A team of engineers from Caltech has recently pioneered a method for inkjet printing specialized nanoparticles, facilitating the mass production of durable wearable sweat sensors.
A wearable sweat sensor based on the core–shell nanoparticle technology. Image Credit: California Institute of Technology
The future of medicine is increasingly focused on personalized healthcare—understanding exactly what an individual needs and delivering the right mix of nutrients, metabolites, and medications to optimize their well-being. Achieving this level of precision requires continuous monitoring of key health biomarkers.
To address this challenge, a team of engineers at Caltech has developed an innovative inkjet printing technique that enables large-scale production of durable wearable sweat sensors. These sensors can track a variety of biomarkers—including vitamins, hormones, metabolites, and medications—in real time, giving both patients and physicians the ability to monitor changes as they happen.
Wearable biosensors incorporating these newly developed nanoparticles have already been used to track metabolites in long COVID patients and measure chemotherapy drug levels in cancer patients at City of Hope in Duarte, California.
These are just two examples of what is possible. There are many chronic conditions and their biomarkers that these sensors now give us the possibility to monitor continuously and noninvasively.
Wei Gao, Professor, Andrew and Peggy Cherng Department of Medical Engineering, California Institute of Technology
Gao is the corresponding author of a paper published in Nature Materials detailing the new technique.
Gao and his team describe the nanoparticles as core–shell cubic structures. These cubes form in a solution containing the molecule the researchers aim to track—for example, vitamin C. As monomers naturally assemble into a polymer, the target molecule (vitamin C) becomes trapped within the cubic nanoparticles.
A solvent is then used to selectively remove the vitamin C, leaving behind a molecularly imprinted polymer shell with cavities shaped precisely like the removed molecules—similar to artificial antibodies that recognize only specific molecular shapes.
In their latest study, the researchers pair these specialized polymers with a nanoparticle core made of nickel hexacyanoferrate (NiHCF). This material undergoes oxidation or reduction when exposed to an electrical voltage in the presence of human sweat or other bodily fluids. Using vitamin C as an example, the NiHCF core remains exposed—and generates an electrical signal—as long as the vitamin C–shaped cavities remain empty.
However, when vitamin C molecules bind to these cavities, they block access to the NiHCF core, reducing the interaction with sweat or other fluids and weakening the electrical signal. By measuring the strength of this signal, researchers can determine the concentration of vitamin C present.
“This core is critical. The nickel hexacyanoferrate core is highly stable, even in biological fluids, making these sensors ideal for long-term measurement,” Gao added.
Gao is also a Ronald and JoAnne Willens Scholar and an Investigator at the Heritage Medical Research Institute.
The new core-shell nanoparticles offer remarkable versatility, enabling the printing of sensor arrays that detect multiple amino acids, metabolites, hormones, or drugs in sweat and other bodily fluids. By using different nanoparticle "inks" within a single array, researchers can create highly specialized sensors for various applications.
In their study, the team printed nanoparticles designed to bind vitamin C alongside others that target tryptophan and creatinine—a key biomarker for kidney function. These nanoparticles were integrated into a single sensor, which was then mass-produced. All three molecules are of particular interest in research on long COVID.
Additionally, the researchers developed wearable sensors incorporating nanoparticles tailored to detect three different antitumor drugs. These sensors were tested on cancer patients at City of Hope, demonstrating their potential for personalized health monitoring.
Gao added, “Demonstrating the potential of this technology, we were able to remotely monitor the amount of cancer drugs in the body at any given time This is pointing the way to the goal of dose personalization not only for cancer but for many other conditions as well.”
The study also demonstrated that these nanoparticles can be used to print sensors designed for implantation just below the skin, enabling precise monitoring of drug levels in the body.
Journal Reference:
Wang, M. et. al. (2025) Printable molecule-selective core–shell nanoparticles for wearable and implantable sensing. Nature Materials. doi.org/10.1038/s41563-024-02096-4